This invention relates to a heating element having positive temperature coefficient (PTC) ceramic heating units used for heating a fluid such as air, and particularly to a heating element which has ring-shaped PTC ceramic heating units having encircling walls disposed one in the other with their encircling walls spaced apart from each other.
It is known that PTC ceramic material made of semiconductive barium titanate is used as a heating element. A very common construction of the PTC ceramic heating element is in the form of a disc which has less heat radiation surface. This construction is not economical since a large number of heating elements is required to produce high wattage. U.S. Pat. No. 3,927,300 discloses a honeycomb-shaped PTC ceramic element which has a high surface to volume ratio and can generate a large amount of heat radiation. However, to manufacture such a kind of PTC ceramic heating element, a complex process is needed to make molds, and the particle size of the raw material for forming the honeycomb ceramic element must be prepared properly to obtain a particular size distribution. Also the extrusion process for forming the honeycomb-shaped structure is difficult and may cause a large percentage of defective products. In application, such a honeycomb structure, in many cases, can not achieve a uniform voltage due to its non-uniform resistance caused by disuniform temperatures throughout its body.
U.S. Pat. No. 4.162,395 discloses a heating element which includes a plurality of PTC ceramic plates each having four corners and each being spaced apart parallelly from the other. Each PTC ceramic plate is provided with two thin-layer electrodes at its two opposite sides, and all the plates are interconnected by four separate metal corner bars of L-shaped cross-section each of which is welded to one of the corners of all the plates, forming a construction having eight corners. A U-shaped plate is used to house all the interconnected plates. While the PTC ceramic plates can be easily formed, the construction having these plates is still disadvantageous due to its limitation to a configuration having eight corners in which each plate has four corners, thereby restricting the selection of different dimensions and shapes to suit different heating devices. In addition, the assembly of parallel PTC ceramic plates requires a housing member to firmly secure them together or to reinforce the construction thereof, and the heat radiation surfaces provided by these parallel PTC ceramic plates are inefficient.